The present invention relates to a door chain device capable of fastening a door chain to a door from the inside to prevent the door from being opened any more than the length of the chain to guard against the illegal entry of any stranger.
A conventional door chain device is usually constructed as follows:
A chain, composed of a limited number of links, is fixed at one end to a retaining means secured to a door frame and has an engaging member at the other end. Said engaging member is integrally formed of a head portion having a slit in which a cross pin is secured for fixing thereon the end link of the chain, a narrow neck portion and a flange of a larger diameter. A door-side locking member secured to the door has a surface plate integrally formed with a frame to form a space between the surface plate and the door's upper surface. The surface plate has a through hole for receiving the flange portion of the engaging member and a slit continuously formed therewith for allowing the narrow neck portion to slide therein. The door-side locking member is secured with bolts to the door.
The door chain can be fastened to the door in such a way that the flanged portion of the engaging member of the chain is inserted in the through hole of the door-side locking member and then its narrow neck portion is slidably fit in the slit of the door-side locking member, thereby the door and door frame are connected by the chain and the door cannot be opened more than the length of the chain even when the door is unlocked.
The chain can be released from the door by reversing the above-mentioned procedure, i.e. the engaging member is moved upward along the slit to the through hole of the door member and then is removed from the engaging member through the through hole by pulling its head portion up at the right angle to the door's surface.
However, the conventional door chain device has the drawback that when the door is opened to the limit the engaging member of the chain can be moved upwards along the slit of the door-side locking member and be pulled out of the through hole, i.e. the chain can be disengaged from the outside of the door.